Formed core sandwich structures are used in turbine, scram-jet, pulse-jet, rocket, and similar engines, as well as in their structures and housings. In such applications, the core material must be lightweight and capable of withstanding engine vibration and heat, as well as shocks from turbulence and landing. The formed structures must also be capable of retaining their strength as they withstand extreme temperatures for extended periods (e.g., 800-1500° F.), along with repeated heating-cooling cycles.
Typically, the materials that satisfy the above requirements do not allow superplastic forming at economically feasible temperatures and pressures, so forming of the core sheet leads inevitably to thinning of the already thin stock material. This thinning, combined with the large tensile forces required to form the desired core shapes, leads to fractures of the core sheet, which consequently limits the depth to which the core sheet can be drawn. Moreover, when a desired depth is achievable, the thinnest region of the formed core material is typically the point of failure, and thus is a factor in determining the ability of the sandwich structure to withstand torsion and bending loads and in determining the volume density of the finished sandwich structure. As a result of the above restrictions, there is a limit to the depths at which core materials can be drawn using known techniques.
Thus, there exists a need for a formed core that is able to resist thinning and fracturing when undergoing large tensile forces during formation of the completed sandwich into the desired shape.